1. Field of the Invention
The present invention generally relates to the art of electronic amplification using transistor circuits, and more specifically to single-ended and differential feedback amplifiers which have very low signal distortion and input shot noise.
2. Description of the Related Art
Voltage feedback amplifiers are used in applications in which high linearity and low distortion are the primary requirements. However, voltage feedback amplifiers are inherently slow in operation due to capacitances of circuit nodes and the requirement that the stray capacitances be charged or discharged in order to change the voltages at the nodes.
Current feedback amplifiers are capable of much faster operation than typical voltage feedback amplifiers since the charging current available for charging the various capacitance within the circuit can be significantly greater. A basic current feedback amplifier is disclosed in U.S. Pat. No. 4,502,020, entitled "SETTLING TIME REDUCTION IN WIDE-BAND DIRECT-COUPLED TRANSISTOR AMPLIFIERS" issued Feb. 26, 1985 to D. Nelson et al. An improved current feedback amplifier which provides reduced distortion is disclosed in U.S. Pat. No. 4,970,470, entitled "DC-COUPLED TRANSIMPEDANCE AMPLIFIER" issued Nov. 13, 1990 to R. Gosser.
Other well known advantages of current feedback amplifiers include fast slew rates and fixed bandwidth vs. gain. Two key disadvantages of current feedback amplifiers are related to their low impedance feedback input node with its intrinsic linearity limits and excessive shot current noise.
A current feedback amplifier of the type disclosed by Nelson and Gosser includes a class AB push-pull input stage in the form of a transconductance amplifier 10 as illustrated in FIG. 1. The amplifier 10 is known in the art as an "A/B cell" and may be designated by the symbol illustrated in FIG. 2. The amplifier 10 per se is disclosed in U.S. Pat. No. 4,780,689, entitled "AMPLIFIER INPUT CIRCUIT", issued Oct. 25, 1988 to K. Saller et al.
The amplifier 10 comprises an NPN bipolar transistor 12 having a collector connected to a positive power supply VCC through a terminal 14 and an emitter connected through a constant current source 16 to a negative power supply VEE through a terminal 18. A PNP bipolar transistor 20 has a collector connected to the power supply terminal 18 and an emitter connected through a constant current source 22 to the terminal 14. An input voltage Vin is applied through a voltage input terminal 24 to the bases of the transistors 12 and 20.
An NPN transistor 26 has a base connected to the emitter of the transistor 20 and an emitter connected to the emitter of a PNP transistor 28. The base of the transistor 28 is connected to the emitter of the transistor 12. The emitters of the transistors 26 and 28 are connected to a current feedback input terminal 30. The collectors of the transistors 26 and 28 are connected to push-pull current output terminals 32 and 34 respectively.
The current sources 16 and 22 cause constant bias currents IBIAS to flow therethrough which set up suitable quiescent bias currents through the transistors 12, 20, 26 and 28. As the input signal Vin changes, the voltages at the emitters of the transistors 12, 16, 26 and 28 follow. More specifically, the voltages at the emitters of the transistors 12 and 16 are one forward-biased diode drop Vbe below and above Vin respectively, and a voltage Vf at the terminal 30 is substantially equal to Vin.
Although the terminal 30 is ostensibly a push-pull voltage output terminal, it functions in the amplifier 10 as a current feedback input terminal, and will be referred to as such since it receives a current feedback input from a source external to the amplifier 10. Since the emitters of the transistors 26 and 28 are connected to the terminal 30, the impedance presented by the terminal 30 to an external signal is low, and current can flow into or out of the terminal 30.
FIG. 3 illustrates a complete current feedback amplifier 40 which is a simplified version of the amplifier disclosed in the Nelson and Gosser patents and includes the transconductance amplifier 10. The current output terminals 32 and 34 are connected through current mirrors 42 and 44 to a node 46. The current mirrors 42 and 44 mirror the currents flowing through the terminals 32 and 34 to the node 46, which has a very high impedance.
An amplifier 50, which is identical to the amplifier 10, serves as a buffer amplifier between the high impedance node 46 and an output terminal 54. A voltage input terminal 52 of the amplifier 50 is connected to the node 46, and the output terminal 54 is connected to the current feedback input terminal 30. Resistors 56 and 58 form a resistive current divider and serve as the current feedback network between the amplifier output 54 and the current feedback input terminal 30.
The amplifier 50 further has terminals 60 and 62 which correspond to the terminals 14 and 32 respectively of the amplifier 10 and are connected to the VCC supply, and terminals 64 and 66 which correspond to the terminals 18 and 34 respectively of the amplifier 10 and are connected to the VEE supply. A capacitor 48 is connected between the node 46 and signal ground and serves as amplifier loop compensation. The amplifier 50 produces an output voltage Vout at the terminal 54.
The amplifier 50 further has terminals 60 and 62 which correspond to the terminals 14 and 32 of the amplifier 10 respectively and are connected to the VCC supply, and terminals 64 and 66 which correspond to the terminals 18 and 34 of the amplifier 10 respectively and are connected to the VEE supply. The amplifier 50 produces an output voltage Vout at the terminal 54. The node 46, capacitor 48 and amplifier 50 constitute a transimpedance amplifier 68.
The current mirror 42 includes a diode-connected PNP transistor 70 and a resistor 72 which are connected in series between the terminal 32 and supply VCC, and a PNP transistor 74 which has a base connected to the base of the transistor 70. The collector of the transistor 74 is connected to the node 46, whereas the emitter of the transistor 74 is connected through a resistor 76 to the supply VCC.
The current mirror 44 includes a diode-connected NPN transistor 78 and a resistor 80 which are connected in series between the terminal 34 and supply VEE, and an NPN transistor 82 which has a base connected to the base of the transistor 78. The collector of the transistor 82 is connected to the node 46, whereas the emitter of the transistor 82 is connected through a resistor 84 to the supply VEE.
Under static conditions, the output voltage Vout has a value which is equal to Vin.times.(1+R56+R58). The resistors 56 and 58 constitute a current divider which causes a fraction of the output current from Vout to appear at the terminal 30. Due to the action of the feedback loop, the voltage at the terminal 30, under static conditions, is equal to Vin.
If the input voltage Vin at the input terminal 24 increases above the voltage at the feedback terminal 30, current will flow out of the feedback terminal 30, with the current through the terminal 32 increasing and the current through the terminal 34 decreasing. These currents are mirrored by the current mirrors 42 and 44 to the node 46, and causes current to flow out of the node 46 and charge the capacitor 48. The voltage across the capacitor 48 increases and causes the output voltage Vout to increase until the voltage at the feedback terminal 30 increases to the new value of Vin. The operation is opposite for a decrease in the input voltage Vin.
The effective width of the base region in a bipolar transistor varies as a non-linear function of the collector-emitter voltage Vce. This causes modulation of the current gain .beta. of the transistor which in turn causes a change in the base and collector currents of the transistor even though the emitter current remains constant. This is known as "base-width" or "Early effect" modulation.
As is evident from viewing FIG. 1, the transistors 12 and 20, and the bases of the transistors 26 and 28 are outside the feedback loop which leads from the output terminal 54 of the amplifier 50 to the current feedback terminal 30 of the amplifier 10. The transistors 12, 20, 26 and 28 are therefore subject to base-width modulation and generate distortion which appears in the output signal Vout. Any parametric asymmetry between these transistors exacerbates the distortion. The lack of feedback to the front end of the amplifier 10 also limits the linearity of the current feedback amplifier configuration.
In addition, current noise generated in the transistors 26 and 28 is summed directly into the feedback signal path terminal 30 and thus leads to excessive amplifier output noise. These drawbacks have prevented current feedback amplifiers from being used in applications in which low distortion and noise are required in addition to high slew rate and high frequency response.